Transfer pumps of the "gear within a gear" configuration are used in many applications to pump fluids at relatively high flow rates at relatively low pressures (less than 1000 psi) as compared to hydraulic system pumps which often operate at pressures well in excess of 1000 psi. This type of transfer pump, usually includes an internal crescent positioned between an outer, driven gear (alternately termed a "rotor") and a smaller, idler gear. The outer gear is connected to a shaft that extends through the housing and is attached directly or indirectly to a drive motor. The idler gear rotates about a fixed idler pin and is driven by the outer gear, as distinguished from a "gerotor" type of gear pump in which an "outer gear" includes inwardly directed teeth that mesh with, and are driven by, an internal drive gear.
A gear within a gear type of transfer pump is called upon to perform a wide variety of tasks in a wide variety of environments. In at least some applications, this type of gear transfer pump is used to pump fluids which contain abrasive material or fluids with particles in suspension. In conventional gear pumps, pumping these kinds of fluids often cause premature failure in one or more pump components. Pump constructions have been disclosed in the past which were intended to be used in applications involving fluids containing entrained solids. An example of such a pump is illustrated in U.S. Pat. No. 1,709,580. According to the disclosure in this patent, the illustrated pump is intended to pump pulp in solution such as paper or wood pulp. In the disclosed pump a shearing action is provided for preventing the rolling and jamming of the pulp between the casing and rotor. A flushing arrangement is also used for flushing the bearings of the idler pinion and rotor shaft to keep them free from accumulation of pulp. The pump disclosed in this patent is not believed to be readily adaptable to applications that involve a pumped fluid that contains entrained abrasives.